Structural gaskets for glazing

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Structural gaskets for glazing

Garden, G. K.

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Structural gaskets for glazing

B Y G . K . G A R D E N N A T I O N A L R E S E A R C H C O U N C I L D I V I S I O N O F B U I L D I N G R E S E A R C H C A N A D A

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Technical Paper No. 262

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September 1968

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Structural

gaskets for glazing

BY G. K. GARDEN,

NATIONAL RESEARCH COIINCIL, CANADA DTVISION OF' BUILDING RESEARCH

Structural neoprene gaskets were adopted by the con-struction industry in North America following their successful use in automobiles. A buildtng, however, is rather different from an automobile, and unfortunately these gaskets have not always performed to the satis-faction of owners, arehitects and builders.

The problems are not generally severe but do consti-tute a nuisance, and there have been a few serious failures.r Penetration of rain is generally slight and the small amount of air leakage normally goes un-noticed. Accumulation of water in the glazing channel, due to partial penetration of rain and condensation associated with air leakage, however, has contributed to the failure of sealed double glazing units. The fracture of glazing unlts due to edge pressure, and failure to retain the glass against wind load, are other problems that have occurred. Although simple in concept, glazirrg with gaskets does present minor fabrication and in-stallation problems.

Despite these problems structural gaskets holcl great appeal. Theoretically, all the requirements of a glazing joint can be met by one element, and it can be installed in a fairly simple operation.

Structural gaskets are made from an extrusion of a relatively hard or stiff elastomer - neoprene. The extrusion cross-section is desigrred in such a manner that only a spreader strip inserted in a special recess in the extrusion is required to provide g:lass retention and weathertightness. Straight lengths of the cxtrusion are factory-joined to form a continuous gasket, which must fit both the glass and the frame to very precise tolerances. Corners are formed by injectlon moulding, which vulcanizes the straight sections together and makes the required features of the gasket continuous. Manufacturing problems have generally been over-come, including fabrication to the precise tolerances required. The need to maintain accurate opening and glass dimensions on the building, however, introduces some difficulties, but filler strips are used to com-pensate for minor variation. Many tricks have been learned that simplify installation of the gaskets and glass, but cutting or ripping of the openable leg of the gasket at a corner stitl frequently occurs, resulting in the necessity to use a liquid sealant to complete the required seal. When cold, the elastomer becomes ex-tremely stiff, but work can continue if the gasket is thoroughly heated and if installation is completed before it cools. Special lubricants that facilitate installation of the glazing unit and the spreader strip have also been developed.

The basic premises on which the desigu of structural gaskets is based are that a perfect seal against air and water leakage can be achieved between the gasket and the surfaces of the elements being joined, and that the loads applied to the glass can be safely transmitted by the gasket to the surrounding structure' To achieve these conditions, a precise balance of mechanical pressures between gasket and glass, and between gasket aud frame must be maintained under all circumstances. To gain the required seal for weathertightness, sealing lips are incorporated at appropriate points on the gasket cross-section. As these sealing lips cannot follow small surface irregularities, it is essential that all surfaces to which the gasket must seal be exception-ally smooth, free of voids, protrusions and even rust or dirt. Even when such good surfaces are provided, the

Fleprint from NBRI Report 51 C "Weathertight ioints for walls'

maintenance of a perfect seal is still not assured. Rota-tional deflection of the gasket when the glass ls subjected to a wind load wilt alter the balance of pressure, and the sealing lips may lose contact with the surfaces of the glass or frame. The surface condi-tions may also be altered by subsequent corrosion or dirt accumulation. Failure to achieve or to maintain a perfect seal is the key factor in the occurrence of raln penetration and water accumulation ln the structural gaskets available at this time.

In transmitting horizontal loads on tJ:e glass to the surrounding frame, a structural gasket tends to rotate or roll out of the frame. The <roll-out> resistance of a gasket is greatest when the clear space (that not occupied by the web of the gasket) between the edges of the glass and the frame member is the least. This is achieved when the glass and frame member fit tightly to the bottom of their respective channels in the gasket. Where edge clearance has occurred because of un-expected construction tolerances and differential move-ments and <roll-out> resistance for the resulting eondi-tions was insufficient, loss of glass retention has occurred. When glass is installed without edge clear-ance, however, edge loading of the glass due to differen-tial movements can result in fracture of the Elazin,g unit. Some gaskets actually rely upon pressure from the edge of the glass to enable them to remain safely in place and to develop the pressure on the sealing lips required for weathertightness. Obviously, these designs do not recogrrize the existence of construction tolerances and the inevitable differential movements between the glass and frame that occur because of temperature changes, deflections under load and other conditions'

Sug g esteil nod,if ications

The least that could be done to improve performance of structural gaskets would be to provide drain holes to prevent the accumulation and storage of water in spaces in the gasket joints. This alone would not be advisable, however, since the drain holes could also provide an easy path for water to enter the spaces during wind-driven rain storms unless the air leakage at the inner seal were reduced to almost zero. This might be possible with the introduction of a mastic or sealant under the inner sealing lips. The suecess of this modification, however, would depend to a high degree upon the sealant and its ability to remain in its proper location, to retain its desirable properties and to keep the air leakage at an absolute minimum despite the rotational deflection of the gasket.

The most desirable change is to remove the necessity for a perfect seal. By designing the gasket on the basis of the <<rain screen>> or <<two-stage weatherproofing> principle, the outer seal of the gasket could be relieved of the air pressure difference that acts to force water inward and the inner seal would only be required to resist air leakage.z Provision of the necessary spaces in the joint for pressure equalization, however, is not corapatible with the glass retention requirement unless the <<roll-out>> resistance of the gasket is improved. As clear space is also reguired in the glazing channel to allow differential movements to occur between the glass and frame and to compensate for construction tolerances, improved <rpll-out> resistance must be sought. There are many reasons why increasing the cross-sectional area of a structural gasket does not

appear to be an acceptable means of increasing <roll-out> resistance. If the gasket were attaehed to the frame of the opening, however, there should be a vast lmprove-ment in its performance. To accomplish this attach-ment, adhesives, mechanical fasteners or interlocks could be employed. Some gaskets can be attached by driving their rtbbed spline portion into a reglet or channel in the frame.

Conclusi,on

Many structural neoprene gasket designs available at present are performing well, but some dlfficulties are being experienced. It is believed that they can be improved by modifications to increase their ability to retain the glass through improved attachment to the frame or changes in their cross-section. Improved weathertlghtness should be possible lf spaces can be incorporated in the Joints that are open to the exterior

to provide equalization of air pressure across the wetted plane and to allow drainage. With some improvement, structural neopreDe gaskets offer a logical system for glaaing.

This paper is a contribution from The Division of Building Research, Natlonal Research Council, Canada, and is published with the approval of the Director of tJte Division.

References

(1) Garden, G. K. Some experiences with joints. CIB Symposium on Weathertight Joints for Walls. Oslo, September 1967.

(2) Garden, G. K. Rain penetration and its control. National Research Councll, Divi,sion of* Buitaing Research, Canadlan Building Dlgest No. 40, Ottawa, Aprtl 1963.

R6sum6

Les Jolats proflds pout vltrages

Les profil6s en n6oprdne sont largement utilis6s par I'lndustrie du bdtiment en Am6rique du Nord, Ils four-nissent un service assez bon, mais difficult6s se produi-sent, qui comprennent I'infiltration de I'eau de pluie, les fuites des vitrages pr6-scell6s, le bris du verre et le d6logement du carreau sous Ia pressioD du vent.

Le maintien du carreau en place par le profilE et I'herm6ticit€ des joints d€pendent d'un €qullibrage pr6cis des pressions d'air entre carreau, profil6 et chdssis; il est ndcessaire que les surfaces en contact soient extr6ment lisses et propres. Si I'une des condl-tions pr6cddentes n'est pas remplie, I'ensemble risque de faire d6faut i court terme. Ces exigences sont trOs

astreigrantes et dEmontrent un manque d'apprdciatlon des possibilit6s r6elles des mat6riaux et des jolnts et de la qualitG de la main-d'oeuvre disponibles.

Le besoin de pr6cision dans I'assemblage des pi0ces d'huisserie ne serait pas aussl pressaDt s'll €tait possible de fixer les profil6s 6lastiques A I'encadrement de fagon ir, r6duire leur ga,uchlssement par rotation et de modifier la forme de leur section transvergale con-form€ment aux principes de l'6tanch6it6 de deux €tapes. Les profilEs en n6opr0ne modifi6s de cette fagon four-niraient le systOme de fixation du vttrage beaucoup plus logique.